Eight fixed energy tariffs will cease at the end of this month, resulting in bill hikes of up to £237.65 for those who don't switch to a better deal. A further five fixed tariffs will expire by 31 October.
Experts at GoCompare.com say that First Utility customers in north-west England, who are currently on the iSave Fixed v7 September 2014 dual fuel tariff, will see the biggest average bill increase at £237.65.
The biggest increase for a big six customer will be for those in south-east England who are on Scottish Power's Online Energy Saver 23 tariff, with an average price hike of £164.93.
Under Ofgem rules energy firms must give at least 40 days' warning that a fixed deal is coming to an end. Customers should act on receipt of the letter as the switching process usually takes about six weeks.
The current cheapest tariff is Extra Energy ValuePlus. It costs an average of £989.57 and is fixed until September 2015, but it comes with an exit fee of £25 per fuel if you switch to a cheaper tariff before the end date.